[SPARK-12094][SQL] Prettier tree string for TreeNode

When examining plans of complex queries with multiple joins, a pain point of mine is that, it's hard to immediately see the sibling node of a specific query plan node. This PR adds tree lines for the tree string of a `TreeNode`, so that the result can be visually more intuitive.

Author: Cheng Lian <lian@databricks.com>

Closes #10099 from liancheng/prettier-tree-string.